Incremental transducer comprising coded track gratings traversed by light rays a plurality of times and using polarized beam splitters



Jan. 13, 1970 F HOCK ET AL 3,489,908

INCREMENTAL TRANSDUCER COMPRISING" coDEn TRACK GRATINGS TRAVERSED BY LIGHT RAYS A PLURALITY OF TIMES AND usme POLARIZED BEAM SPLITTERS Filed July 14, 1967 2 Sheets-Sheet 1 Fig. i E 27 30 l 2/ q 35 Fig. 3

ATTORNEYS Jan. 13, 19% HQCK ET AL INCREMENTAL TRANSDUCER COMPRISING'CODED TRACK GRATINGS TRAVERSED BY LIGHT RAYS A PLURALITY OF TIMES AND USING POLARIZED BEAM SPLITTERS 2 Sheets-Sheet 2 Filed July 14. 1967 M 0 H m0 WW WM M F N N A M W E H T W K mwmw/s 15, Cl. 250-237 5 Claims ABSTRACT OF THE DISCLOSURE In a photoelectric incremental transducer comprising a grating of which an image is projected onto a second grating or onto itself and further comprising amongst other elements a light source, a beam splitter, and at least two photoelectric receivers for generating position defining signals a device is disclosed for generating additional signals which may be applied as control signals to various usages, e.g. for controlling the working accuracy of a counter mechanism connected to said in cremental transducer.

Said device comprises an index grating disposed be hind said beam splitter in the direction of the light fiux and having transparent and non-transparent lines thereon. Two additional photoelectric receivers are employed behind said index grating, which receivers are adapted to scan said index grating, thereby generating additional signals.

Further, gratings are disclosed which are provided with code tracks extending in parallel to the lines on the grating. Of said code tracks no images are projected onto the second grating or onto themselves, However, images of said code tracks are projected onto corresponding code tracks on the index grating. From the photo electric receivers scanning said index grating code signals are then obtained which may be used for a coded defining of a position,

CROSS REFERENCE OF RELATED APPLICATIONS Priority is claimed under 35 U.S.C. 119 for the in vention as defined in claim it from application L 54,090 IXb/42d, filed July 16, 1966, in the Patent Office of the Federal Republic of Germany. No foreign priority is claimed for the invention defined in claims 2 through 5.

BACKGROUND OF THE INVENTION Field of the invention The invention relates to a measuring apparatus for determining the length or the angle of an object. More specifically, the invention relates to a measuring device which comprises a light source, an optical system, a grating which is displaceable relative to said optical system, and photoelectric receivers, by the displacement of said grating-which may be a linear or a rotary motionthe light flux is caused to vary periodically which variations are converted to photoelectric current fluctuations and may be used in electronic counting techniques or analogus evaluations.

Description of the prior art Measuring devices of the above described type are known to those skilled in this particular art as photoelectric incremental transducers. They may be used for measuring the extent of the motion of the grating relative to an object, thereby defining the length of said object. According to the same principle an angle may be measured or the angular position oi an object may be determined by a device commonly called incremental angle transducer, wherein said grating is a disc having radially extending lines and wherein the motion of said grating is a rotatory one It is an object of the present invention to provide a. means adapted to generate additional signals which may be used for controlling purposes of various kinds.

It is further object-of the invention to provide means by which code signals may be obtained for coded defining of a position.

SUMMARY OF THE INVENTION Photoelectric incremental transducers as known in the art comprise a displaceable grating, an image of a portion of said grating is projected onto itself or onto a second grating which is rigidly connected to the first one, The elements conventionally used for this imaging purpose comprise a light source, a first objective, a first reflecting element, a beam doubling and polarizing element, a second and non-poarizing reflecting element and a second objective. Behind the grating in the direction of the light flux a polarizing beam splitter is arranged by which the two polarized light beams are transmitted or reflected respectively to at least two photoelectric receivers. Said receivers being adapted to generate positiondefining signals. According to the invention a non-polar izing beam splitter is used for the above mentioned second and non-polarizing reflecting element which transmits part of the impinging light to an additional stationarily mounted index grating. A po'arizirg beam splitter is mounted behind said additional index grating, also being adapted to transmit or reflect respectively said two polarized beams onto photoelectric receivers. By the latter additional signals are generated which may be made use of in the aforedescribed manner.

Since the additional index grating is stationarily mounted, the frequency of the additionally generated signals will be only half of the frequency of the positiondefining signals.

Further, by specially designing the employed gratings it is also possible to obtain sigrals having the frequency f, 2 and 4f, wherein 4f stands for the frequency of the position-defining signalsi DESCRIPTION OF THE DRAWINGS FIGURE 1 shows schematically a photoelectric incremental transducer of known design and being provided with an additional grating according to the invention;

FIGURE 2 shows schematically a photoelectric incremental angle transducer having a radial grating and an additional grating and further being provided with code tracks extending in parallel to the lines of the gratings;

FIGURE 2a is an enlarged portion of the grating used in the embodiment depicted in FIGURE 2;

FIGURE 2b shows a modification of the photoelectric incremental transducer according to FIGURE 2;

FIGURE 3 shows schematically a photoelectric incremental transducer in which a first phase grating is imaged on a second phase grating, and further two additional gratings are employed one of which also being a phase grating, the other one being an amplitude grating.

DESCRIPTION OF THE PREFERRED EMBODIMENTS In FIGURE 1 a light source 5 is depicted which illuminates by means of a condenser 6 a portion of a transparent linear grating 8. The light source 5 is preferably provided with an incandescent filament forming a linear uniting surtace. (hating t is provided with non-translines .1 int: on, to which the emitting surface of extends in parallel and between which liner, 1' the emltted by source d pass to an objective 9, a 'lprism lb, and to a Vtollaston-prism .20. [is is well m to those skilled in the art a llollaston-prism douthe impinging light beam, both light beams then con "ring in their path relatively inclined to each other by l angle which dc ends on the properties; oi. the Vi Olltw prism. Both beams then also being polarized in di' .cctions normal to each other.

The two beams impinge on a non-polarizing beam-splitting cube it A portion of said beams is reflected to an objective l2 and further to the grating 8. In this way two "rages of the illuminated grating portionsslightly dised relative to each otherare projected back onto the grating, the imaging ratio of the employed objectives and if; being, of course, exactly 1:1.

Both light beams transmit the grating it in the space etween the. lines 7 and travel to a polarizing beam splitter a. The latter being adapted to let pass those light beams vluch are polarized in one direction through a lens 15 to it first photoelectric receiver 17 and to reflect the light beams which are polarized in the other direction through a. lens 14 to a second photoelectric receiver to. The signals generated by said photoelectric receivers l6, l7 constitute the position-defining signals and are conducted to an evaluation device 18.

The elements described hereinbefore are known in the incremental transducer technique and, therefore, belong to the prior art, except beam-splitting cube it which previously used to be a simple non-polarizing reflecting elet ent.

According to the invention this element has been replaced by said beam-splitting cube 11 through which :1 portion of each polarized light beam originating from the Wollaston-prism 20 travels to a lens 21 and onto an index grating 22 which is stationarily mounted in the path of said beams. Behind grating 22 a second polarizing beam-splitting cube 23 is disposed which corresponds in design and effect to the first mentioned cube 13. It separates the two polarized beams according to their direction of polarization, allowing one beam to pass on through lens 26 to the photoelectric receiver 2 and reflecting the other beam through lens 27 to photoelectric receiver in this manner a device is provided by which two sigi is are obtained in addition to the position-defining tlEllS generated by the receivers to ant 17. However, since grating 1 1 is stationarily mounted, in contradistinction to this part of grating 8 onto which. the illuminated portion at grating 8 is projected. the frequency of the additionally aerated s gnals will he only half the frequency of the aeration-defining signals.

The additionally generated signals may be used for various purposes. For example, an auxiliary counter may be operated by said signals. The reading of this auxiliary counter may be compared with the reading of the counter storing the result of the measurement. Thereby it will be possible to detect mistakes in the result or" the measure ment immediately and to signal and/or automatically correct said mistakes.

A second embodiment of the invention is depicted by way of example in FlGUR'tE 2. The grating 38 in said embodiment is a circular disc having radially extending lines thereon. Grating is rotatable about axis 39. The r eyice, therefore, is an incremental angle transducer, wherein, however, the elements 5', 6i 9, ill. 18 and corresponds to the like numbered elements as described above with reference to the embodiment according to FIGURE 1.

Besides. with the lines 37 the grating Ju is in addition provided with the tracks 4t, One track may constitute, for example, a binary code, other tracks may be used to define other types of codes (FIGURE 2a). The ndex. grating 22a is provided with a corresponding arl angement of grating lines and tracks. Behind the index rating in the direction of the light flux a polarizing beam splitting cube is arranged upon which those light beams npinge which have transmitted the lines 37 of the grating .38. Cube 23 either transmits or reflects respectively the ight beams according to the direction in which they are l polarized through lenses 26 or 27 to the additional signalgenerating photoelectric receivers I? For scanning the tracks ill, ll. either also a polarizing beam splitting cube -l5 may be employed with photoelectric receivers 4-6 and 47 (FIGUR 2), or optical fibers 43 may be used for conducting the light onto rcceivers 44 (FIGURE 21)).

Since the frequency of the signals generated by the receivers 24-, 2.5 is only half of the frequency of the signals obtained from receivers in and ft! they are suitable for coded representation of the measurement values. From the tracks 40, lll, 42 additional code signals may be obtained. Thus, according to the invention from the tracks of the grating lines the same number of informations may be obtained as was previously possible only by using two code tracks.

It will be possible, of course, to employ a grating having additional code tracks as described with reference to FIGURE 2 in a linear measurement device according to FIGURE 1. Also, no difliculties will be encountered in assembling the index gratings from two single parts one of which being provided with. the grating lines, the other one with the code tracks.

In FIGURE 3 an embodiment of the invention is illustrated wherein the light source 5 through condenser 6 illuminates a portion of a phase grating By means of the objective 9, the beam doubling and polarizing element 26*, the pentaprism it), beam splitting cube Ii, and objective 12 two images of this portion are projected onto a second phase grating 31 which is rigidly connected to the first mentioned phase grating 30. The scale ratio of object and images being 121.

A polarizing beam splitting cube i3 is disposed behind phase grating 31, said cube being adapted to separate the counter-phase modulated images according to their diffraction order and to conduct said images through lenses 5 to photoelectric receivers Behind beam splitting cube Itlt a reflecting prism 34 is arranged with the lens Zlt inserted between said elements. The reflecting prism 34 is adapted to reflect a portion of the impinging beams to a phase grating Photoelectric receivers 51', 52 are attached to said grating 35 by means of a polarizing beam splitting cube 3d and lenses 5t).

To one surface of pentaprism lit a complementary prism ltla is cemented, thereby turning said pentaprism s rface into a beam splitting surface. A portion oi the ht impinging on pentaprism til transmits said beam splitting surface and passes through a phase retarding plate dtl disposed behind complementary prism ifla.

Plate titl effects the various diffraction orders of the light in the same way as is well known from phase cOn- 'trast microscopy. An objective is provided behind plate 60. Said objective projects two amplitude images of the phase grating 30 onto an amplitude grating 27:: having a grating constant equal to the grating constant of the projected grating images, which grating 27a disposed behind objective at in the direction of the travelling light.

Further, a polarizing beam splitting cube is em ployed which transmits or reflects respectively the light beams through lenses 26, 27 to two photoelectric receivers 2d. 25. The frequencies of the signals generated by the photoelectric receivers 24, 225", by the receivers 51, 52, and by the receivers 32, 33 are of the ratio l :2:4.

It is thus readily discernible that by employing the means according to the invention from the motion of a single grating a plurality of signals may be produced the frequencies of which are of a fixed ratio and which may be used for various purposes as indicated hereinbefore.

What is claimed is: 1, A photoelectric incremental transducer comprising a partially transparent grating (8, 30, 38) having equidistantly positioned non-transparent lines (7, 37) thereon, said grating being displaceable;

a light source (5) adapted to illuminate part of said grating, said light source being provided with an incandescent filament having a. linearly extending emitting surface;

optical means (9, 12) adapted to project two a light source (5) adapted to illuminate part of said 5 images of said illuminated part of said phase gratgrating; 7 ing (30) onto a-second phase grating (31) rigidly optical means (9, it 12) adapted to project an image connected to said first phase grating (30), said optiof said illuminated part of said grating onto a seccal means further including an optical element (20) 0nd part of said grating, said optical means further 10 disposed on the opposite side of the grating from including an'optical element (20) disposed On the where said light source is arranged, said optical eleopposite side of the grating from where. said light e t being adapted to double the light beam passing source is arranged, said ptical el m n being through said grating, thereby polarizing each of said adapted to double the light beams passing through beams in directions normal to each other; said grating-thereby polarizing each of Said beams a non-polarizing beam splitting element (11) in the in directions normal to each other; path of said light rays and being adapted to project a non-polarizing beam splitting element (11) in the said doubled. light beams onto said phase grating path of said light rays and being adapted to project (31); s id doubled light beams back Onto Said grating a polarizing beam splitting element (13) being dis- 30, 38); posed behind said second phase grating (31) in the a polarizing beam splitting element (13) being disdire tio f th light flux;

Posed behind Said grating in the direction two photoelectric receivers (16, 17, 32, 33) to which of the light flux; said beam splitting element (13) transmits or retWO photoelectric receivers Which flects respectively'one of said double beams, and

Said beam Splitting element trammits or being adapted to generate two position-defining phofiects respectively one of said double beams, and toelectfic t i l being adapt d to generate tWO POSiliOWdefining a third phase grating (35) being disposed behind said photoelectric current Signals; non-polarizing beam splitting element (11) in the a second and stati n rily mounted grating (22, 1 travel direction of one of the two light portions 22b) being disposed behind said beam splitting eleemerging f id element (11); Iheht i the direction of the light fl a second polarizing beam spliting element (36) being a second polarizing beam splitting element (23) being di d b hi d id phase grating disposed behind Said Second grating photoelectric receivers (51, 52) onto which said polarphotoelectricreceivers (24, 25) onto which said second izing b splitting element (36) transmits or re- Polarilhlg beam Splitting element transmits 0f 35 fiects respecitvely the light beams which have passed reflects respectively this portion of the-light beams through said phase grating (35); Wh Passed through Said sficond grating a complementary prism (100) being cemented to the 22a, 22b) said photoelectric receivers (24, 25) genpentaprism (10); crating Signals of half the frequency f the Signals a phase retarding plate (60) in the path of the light generated by said first mentioned photoelectric re- 40 rays passing through id prism (10 ceivers (16, 17). an objective (61) adapted to project two amplitude 2. A photoelectric incremental transducer according to images of Said ha grating 30 Onto an jiclaim 1 and further comp i i g tude grating (22b) disposed behind said objective code tracks (40, 41, 42) on the grating (38) extending (61);

in paral l t lines of Said grating; 45 a third polarizing be'am splitting element (23) being code tracks on the second grating (22a) extending in di d b hi d idr ti (22b);

Parallel to the lines of Said grating and CorreSpOnd' photoelectric receivers (24, 25) onto which said third ing in graduation to the code tracks on the first menpolarizing b litti g element 3 transmits or ti n g g reflects respectively this portion of the light beams Optical meahs being disposed behind Said grating 5@ which has passedz-through said amplitude grating said means being adapted to conduct the light beams (22b), passing through said code tracks to photoelectric R f Cit d receiverso 3., A. photoelectric incremental transducer according UNITED STATbS PATENTS to claim 2, wherein said optical means comprise 2,349,912 9/1958 PlBSSc t ah 250220X a polarizing beam splitting element (45) for trans- 2,986,066 5/1961 Rouy 356-416 mitting or reflecting respectively said light beams to 2,998,746 9/ 1961 Gievers 25O-225 X twophotoelectric receivers (46,47) 3,198,061 3/1965 Hock 250237 4, A photoelectric incremental transducer according ,4 1/1969 Hock 250231 to claim 2, wherein said optical means comprise 6 optical fiber elements (43) corresponding in number to the number of code tracks on the grating, said fiber elements being adapted to conduct said light beams to photoelectric receivers (44).

5, A photoelectric incremental transducer comprising a a partially transparent phase grating (30), said grating being displaceable;

JAMES W. LAWRENCE, Primary Examiner V. LAFRANCHI, Assistant Examiner US. Cl. XR 

